ME SHELL - thats the sort of story that worries us!!!!...i mean, if she was visiting her fiance and not planning on getting married on that particular visit then i dont know what the problem was, she was visiting like my fiance will be so we should NOT need a fiance visa. The UKvisa website is very vague about visiting fiances.
Was there any weird circumstances for your friend like garry mentions
? How long did she say she was staying for 
MY GF wears a 3carat rock on her finger, the I/O is bound to spot it - i told her not to hide it and be truthfull.
As much as none of us likes it, some IOs wont care that you say you have no intentions of getting married on 'this' visit....even if you are telling the truth :/ The fact is eventually you plan to be together forever..they may decide you will think you cant bear to be apart any longer and will do whatever it takes.b
Its a double edged sword, you need enough ties to keep you in the US until you are in the UK on a settlement visa (if thats the route you take) but yet you need a good enough reason to be in the UK.
Like someone else mentioned yes they do judge a book by its cover

I dont have the greatest circumstances here in the US, I dont have a job, I am getting divorced, have a young son and am young myself...I didnt plan to overstay or settle illegally, but I was denied entry anyway. Because they felt I didnt have enough ties to the US. They decided that no matter what I said I was lying. Is it fair? Not really, but theres nothing I can do about it. We just had to decide what the next step was (we were not engaged at that time) and go from there. At least soon it will all be over for us, I'll be over there on a fiancee visa asap, can wear my ring and not care who sees it, and no ones gonna accuse me of trying to settle illegally

Again I am not trying to rain on anyones parade, some never have problems and then there are those that have nothing but. And theres all kinds of in betweens.
I am going by my own experiences..but even then I have noticed it seems no one can really agree, the airport staff, other IOs, etc all tell me I shouldnt have been denied a visa OR entry, but there was an ECO and an IO that didnt feel that way, and unfortunately they were the ones that mattered.